This invention relates to the field of communications, and, in particular, to optical devices for satellite communications systems.
Orbiting satellites are an important aspect of modern communication systems. Originally used for xe2x80x9csingle-bouncexe2x80x9d communication, with a signal going up from one place on the surface of the earth and coming down in another, communication satellites are now being used to form complex networks in space, with each satellite in the network being able to communicate with many, but not all, of the other satellites. Optical intersatellite links, with their high directionality, high energy efficiency, and tremendous information bandwidth, allow satellites to talk to one another, and to transmit a much larger amount of information. Optical FM links, a new concept, offer a way to transmit not only digital signals, but also analog signals, and to do this with a much higher signal quality than was heretofore possible. However, effective optical FM links have not yet been demonstrated primarily because of the lack of appropriate photonic components, such as FM optical sources, limiters, and discriminators.
The only experimentally-proven discriminator that has appeared in the literature to date is a Mach-Zehnder interferometer with a delay line in one branch, as discussed in the Journal of Lightwave Tech., Vol. 10, 787-793, 1992 article by W. V. Sorin, et al, entitled xe2x80x9cFrequency Domain Analysis of an Optical FM Discriminatorxe2x80x9d. However, the disadvantages of this approach are twofold. First, the interferometer has a sin xcex94f response, instead of a desired xcex94f. Although approximately linear for small frequency deviations, the response becomes nonlinear for larger frequency deviations. Secondly, the delay line length, which can be several centimeters, must be held constant to a small fraction of an optical wavelength. This requires active and precise thermal control of the circuit. The size and weight of these controllers, and the added complexity associated with them, are undesirable features for space-borne (e.g., satellite communication) systems, where size and weight are of paramount importance.
U.S. Pat. No. 5,742,714 issued to Byron teaches interferometrically combining two optical waves to produce an output intensity that can vary from zero (complete destructive interference) to unity (complete constructive interference). The variation of this intensity with wavelength gives a raised cosine response, identical to that produced by the delayxe2x80x94line discriminator of Sorin referenced above. In fact, the Byron device is an example of a delay line discriminator, with the delay being produced by the different positional reflections in the longitudinal filters. While the Byron device uses light, splitters and filters, the device cannot provide the high-linearity needed for the high-fidelity transmission of analog signals. The nonlinearity introduced by the Byron and other interferometric discriminators produces harmonic and intermodulation distortion that can seriously degrade signal quality.
Therefore, to help further realize optical FM inter-satellite links, there exists a need for an effective photonic device that allows one to directly convert excursions in optical frequency into, variations in photocurrent, without the use of optical interferometry. The present invention provides a solution for such need by providing an optical FM discriminator with high linearity characteristics.
In accordance with the present invention, embodiments of an FM discriminator are provided which are highly linear with respect to frequency deviation. Such high linearity is of considerable importance for analog optical links since, as noted earlier, non-linearities introduce undesired harmonic and intermodulation distortion products which, being a form of noise, reduce signal quality. Further, the device in accordance with the present invention does not need the thermal control circuits and elements required by existing interferometric techniques.
In accordance with a preferred embodiment of the present invention a method and apparatus for optical frequency modulation discrimination is provided. An optical beam having a frequency xcfx89 is split into a first optical beam having a frequency xcfx89 and a second optical beam having a frequency xcfx89. A first semiconductor photodetector is provided, the first photodetector providing a first current I1 responsive to the first optical beam input thereon, the first photodetector having a first photodetector spectral response and being biased such that the first current I1 is in a first direction. A second photodetector is also provided, the second photodetector providing a second current I2 responsive to the second optical beam input thereon, the second photodetector having a second photodetector spectral response and being biased such that the second current I2 is in the first direction. An input of a transimpedance amplifier is coupled to an output of the first photodetector and to an input of the second photodetector to provide an output of the transimpedance amplifier proportional to the difference between the first current I1 and the second current I2. A first optical filter is provided to receive the first optical beam prior to incidence upon the first photodetector and a second optical filter to receive the second optical beam prior to incidence upon the second photodetector, wherein the first photodetector spectral response and the second photodetector spectral response are each much broader than respective passbands of the first optical filter and the second optical filter, thus providing photocurrent vs. optical frequency characteristics determined by the respective first optical filter and the second optical filter. The first optical filter and the second optical filter, respectively, can provide Lorentzian line shapes. The first optical filter and/or the second optical filter are tuneable to adjust wavelength separation between the first tunable optical filter and the second tunable optical filter to obtain maximum linearity. The first optical filter and/or the second optical filter are tuneable to operate over a broad range of operating wavelengths. The first optical filter and the second tunable optical filter can be fiber-optic tunable Fabry-Perot filters and the first photodetector and the second photodetector can be high-speed semiconductor photodetectors, such as InGaAs semiconductor photodetectors.
In accordance with an alternate embodiment of the present invention, a method and apparatus for optical frequency modulation discrimination is provided wherein an optical beam having a frequency xcfx89 is split into a first optical beam having a frequency xcfx89 and a second optical beam having a frequency xcfx89. A first photodetector is provided having a peak resonant absorption xcfx891, the first photodetector providing a first current I1 responsive to the first optical beam input thereon, the first photodetector being biased such that the first current I1 is in a first direction. A second photodetector is also provided having a peak resonant absorption xcfx892, the second photodetector providing a second current I2 responsive to the second optical beam input thereon, the second photodetector being biased such that the second current I2 is in the first direction. An input of a transimpedance amplifier is coupled to an output of the first photodetector and to an input of the second photodetector to provide an output of the transimpedance amplifier proportional to the difference between the first current I1 and the second current I2.